The present invention relates to an ion selective electrode and more particularly to an improvement of a tip member for use in an ion selective electrode of a liquid-membrane type in which the tip supports a porous membrane impregnated with an ion-exchanger liquid for exchanging a specific ion at an interface between two immiscible phases, i.e., the ion-exchanger liquid and a solution to be tested.
Such ion selective electrode is shown in FIG. 1 and comprises an outer tubular body 1 and an inner tubular body 2 which are formed integrally with each other. The end portion of the outer tubular body 1 is threadingly engaged with an annular tip 3 and a porous membrane 4 is clamped and held between the lower end of the inner tubular body 2 and the tip 3 to define two reservoirs which are adjacent to the membrane 4. The outer reservoir contains an ion-exchanger liquid 5 for sensing a specific ion therein and the inner reservoir contains an internal reference solution 6 therein. In the inner reservoir there is also provided with an inner electrode 8 of AgCl or the like which is connected to a shielding wire 7 led in from the other end portion of the outer tubular body 1 and which is immersed in the internal reference solution 6. The outer end of the outer tubular body 1 is threadingly engaged with a cap 9.
In the ion selective electrode shown in FIG. 1 the porous membrane 4 holds the ion-exchanger liquid 5 and the internal reference solution 6 and defines an interface between a solution to be tested (not shown) and the liquids 5 and 6. The porous membrane 4 is also formed by a porous material which has a limited size of meshes thereof, such as plastic film, ceramic or the like. The porous membrane 4 is further held tightly by the inner tubular body 2 and the tip 3 in order to prevent any small current leakage. The porous membrane 4 is further more formed by a hydrophobic material or subjected to hydrophobic coating treatment so that the porous membrane 4 is effectively impregnated with the organic ion-exchanger liquid 5 and is not impregnated with the internal reference solution 6. The solution to be tested forms substantially together with the ion-exchanger liquid 5 an interface therebetween so that the electric resistance between the inner electrode 8 and a reference electrode (not shown) depends on the area and the thickness of the porous membrane 4 and becomes very high such as 10.sup.9 .OMEGA..multidot.cm or more. Therefore, it is preferable to use plastic material having high insulation resistance as the outer tubular body 1 and the tip 3 which are in contact with the test solution.
Such an electrode forms a clearance between the plastic tip 3 and the porous membrane 4 by the temperature changes of the test solution and the measuring atmosphere and the swell of the electrode by moisture absorption, so that the current leakage becomes large resulting in worse of performance and it is liable to enter air bubbles in the clearance resulting in inferior durability. In case of designing an electrode for effecting stable operation over a long period of time against the temperature change and the swell by moisture absorption by tightly holding the porous membrane to the same electrode as that shown in FIG. 1, the outer diameter of the electrode must be made large and thus it is necessary to increase the length d of the aperture 10 of the tip 3 which performs ion-exchange to the test solution in strength. Such construction must be used in large amount of the test solutions and it is liable to enter the air bubbles in the aperture 10 at measuring so that normal measurement can not be performed.